Source : Webster's Revised Unabridged Dictionary (1913)

Plesiosaurus \Ple`si*o*sau"rus\, n.; pl. {Plesiosauri}. [NL.,
   fr. Gr ? near + ? a lizard.] (Paleon.)
   A genus of large extinct marine reptiles, having a very long
   neck, a small head, and paddles for swimming. It lived in the
   Mesozoic age.

Source : WordNet®

plesiosaurus
     n : extinct marine reptile with a small head on a long neck a
         short tail and four paddle-shaped limbs; of the Jurassic
         and Cretaceous [syn: {plesiosaur}]